Display unit and electronic apparatus

ABSTRACT

The invention provides a display unit that displays images on both the front and the rear surfaces and that displays images brightly without increasing the electrical power consumption. A liquid crystal display unit includes two liquid crystal cells, a backlight, which is interposed between the two liquid crystal cells and which emits light to the liquid crystal cells, and two reflective polarizers interposed between the backlight and each of the liquid crystal cells, respectively.

BACKGROUND OF THE INVENTION

This is a Divisional of application Ser. No. 11/221,835 filed Sep. 9,2005, which in turn is a Divisional of application Ser. No. 10/678,185filed Oct. 6, 2003 (now U.S. Pat. No. 7,106,396 issued Sep. 12, 2006).The disclosure of the prior applications is hereby incorporated byreference herein in its entirety.

1. Field of Invention

The present invention relates to a display unit and an electronicapparatus. More specifically, the invention relates to a display unitthat can display images on both the front and the rear surfaces.

2. Description of Related Art

The related art includes display units that have a backlight as a lightsource interposed between two liquid crystal panels and that displayimages on both the front and the rear surfaces. These related artdisplay units are disclosed in Japanese Unexamined Patent ApplicationPublication No. 10-90678, and Japanese Unexamined Patent ApplicationPublication No. 2001-290445.

SUMMARY OF THE INVENTION

For the above display unit, a polarizer is disposed between each of thetwo liquid crystal panels and the light source, allowing only lightpolarized in a predetermined direction to enter the display panels. Inthis case, light polarized in a direction that does not match thedirection of the polarization axis (the transmissive axis) of thepolarizer is absorbed by the polarizers. For this reason, the amount oflight that may be used for the display is greatly reduced, causing thedisplay to become dark. Therefore, to obtain a bright display, it isnecessary to use a brighter light source. As a result, the electricalpower consumption increases.

The present invention provides a display unit that displays images onboth the front and the rear surfaces, while addressing or solving theabove, by effectively using the backlight to offer a bright displaywithout causing an increase in the electrical power consumption. Theinvention also provides an electronic apparatus that includes thisdisplay unit.

To address or achieve the above, the display unit according to thepresent invention includes two transmissive polarization axis rotatingdevices to vary the polarization axis of the transmitted light, anillumination device that emits light to each of the transmissivepolarization axis rotating devices and that is disposed between the twotransmissive polarization axis rotating devices, and two reflectivepolarized light selecting devices, each disposed between theillumination device and each of the two transmissive polarization axisrotating devices. The illumination device transmits the reflected lightfrom the two reflective polarized light selecting devices.

In such display unit, among the light emitted from the illuminationdevice, only the transmitted light (first polarized light) selectivelytransmitted through the reflective polarized light selecting devices(first reflective polarized light selecting device) enters thetransmissive polarization axis rotating devices. On the other hand, thereflected light (second polarized light) polarized in a predetermineddirection at the first reflective polarized light selecting deviceenters the reflective polarized light selecting devices on the oppositeside (second reflective polarized light selecting device) via theillumination device. By selectively transmitting the reflected lightthrough the second reflective polarized light selecting device, thereflected light may enter the transmissive polarization axis rotatingdevices on the opposite side. More specifically, by configuring thesecond reflective polarized light selecting device so that it transmitsthe second polarized light while reflecting the first polarized light,the polarized light reflected at one of the reflective polarized lightselecting devices is selectively transmitted through the otherreflective polarized light selecting device. Then this polarized lightenters the transmissive polarization axis rotating devices and may beselectively emitted to the outside from the transmissive polarizationaxis rotating devices, allowing the light emitted from the illuminationdevice to be used for display without any loss. Thus, for this displayunit, which may display images on the front and the rear surfaces, abright display may be obtained by effectively using the light withoutcausing an increase in the electrical power consumption. The reflectivepolarized light selecting devices may include materials, such as acomposite of several different types of birefringent polymeric filmsalternately stacked in layers or a film utilizing the effect of circulardichroism of a cholesteric liquid crystal. Examples of the abovereflective polarized light selecting devices are optical films, such asDBEF (product name) offered by the 3M Company or NIPOCS offered by NittoDenko Co., Ltd.

The reflective polarized light selecting devices reflect light linearlypolarized in a predetermined direction while transmitting light linearlypolarized in a direction that intersects the reflected polarized light.The two reflective polarized light selecting devices may be arranged sothat their reflective polarization axes intersect each other. Thesereflective polarized light selecting devices that selectively reflect ortransmit light according to the direction of their polarization axes maybe used. In such a case, by arranging the reflective polarization axesof the reflective polarized light selecting devices to intersect eachother (preferably at a 90° angle), the reflected light from one of thereflective polarized light selecting devices may be efficientlytransmitted through the other reflective polarized light selectingdevice.

Furthermore, the reflective polarized light selecting devices, whichselectively polarize light based on the difference in the direction ofthe polarization axis, may have an absorptive polarized light selectingdevice that absorbs the polarized light with a polarization axis in apredetermined direction (also referred to as the absorptive polarizationaxis) while transmitting the polarized light with a polarization axisintersecting the absorbed polarized light (also referred to as thetransmissive polarization axis). This absorptive polarized lightselecting device is interposed between the reflective polarized lightselecting devices and the transmissive polarization axis rotatingdevices. In such a case, the reflective polarization axis of therespective reflective polarized light selecting device and thetransmissive polarization axis of the respective absorptive polarizedlight selecting device may be arranged so that they intersect eachother. In other words, at the absorptive polarized light selectingdevices, by once again selectively polarizing the light polarized at thereflective polarized light selecting devices, the polarizationselectivity of the transmissive polarization axis rotating devices forthe incident light becomes higher, enhancing the display quality of thelight emitted from the transmissive polarization axis rotating devices.In particular, since the absorptive polarized light selecting devicegenerally has a higher polarization selectivity in comparison to thereflective polarized light selecting devices, interposing the absorptivepolarized light selecting device between the reflective polarized lightselecting devices and the transmissive polarization axis rotatingdevices increases the polarization selectivity for the light enteringthe transmissive polarization axis rotating devices.

The reflective polarized light selecting devices may be reflectivecircularly polarized light selecting devices including a cholestericliquid crystal, which utilizes the effect of circular dichroism toreflect the circularly polarized light with a predetermined rotativedirection while transmitting the circularly polarized light with theopposite rotative direction. In this case, the two reflective polarizedlight selecting devices (the reflective circularly polarized lightselecting devices) should be arranged so that they each reflectcircularly polarized light with the same rotative direction.

The reflective circularly polarized light selecting devices including acholesteric liquid crystal reflect and rotate the rotative direction ofthe circularly polarized light that enters with a predetermined rotativedirection (a first rotative direction) while transmitting the circularlypolarized light that enters with a different rotative direction (asecond rotative direction). In the present invention, the rotativedirection of the light is determined based on the direction in which thelight travels.

Thus, by arranging two of the above reflective circularly polarizedlight selecting devices so that they both reflect the circularlypolarized light having the same rotative direction (for example, theyboth may reflect the circularly polarized light with the first rotativedirection), the circularly polarized light with the first rotativedirection is reflected at one of the reflective circularly polarizedlight selecting devices and its rotative direction is converted into thesecond rotative direction. Then this polarized light is transmittedthrough the other reflective circularly polarized light selecting deviceand may be emitted to the transmissive polarization axis rotatingdevices.

More specifically, by using two reflective circularly polarized lightselecting devices that each reflect circularly polarized light with thesame rotative direction, the light reflected at one of the reflectivecircularly polarized light selecting devices may be transmitted throughthe other reflective circularly polarized light selecting device, asdescribed above.

For the above reflective circularly polarized light selecting devices, aλ/4 retardation film is interposed between the reflective circularlypolarized light selecting devices and the transmissive polarization axisrotating devices to convert the circularly polarized light with apredetermined rotative direction that has been transmitted through thereflective circularly polarized light selecting devices into linearlypolarized light that is parallel to the transmissive polarization axisof the transmissive polarization axis rotating devices. Then, thislinearly polarized light is selectively emitted while its polarizationaxis is rotated at the transmissive polarization axis rotating devices.

Disposing a scatterer between the reflective polarized light selectingdevices and the absorptive polarized light selecting device suppressesthe reflection at the front surface of the reflective polarized lightselecting devices, and, thus, specular reflection is reduced orprevented.

The illumination device includes a light source and a light guidingplate, which guides the light from the light source. Each of the twotransmissive polarization axis rotating devices is illuminated withlight from both sides of the light guiding plate. The illuminationdevice has a simple structure to effectively shine light onto each ofthe transmissive polarization axis rotating devices.

The illumination device may include a transparent planar light source.This illumination device is much simpler compared to the aboveillumination device, and may effectively shine light onto each of thetransmissive polarization axis rotating devices.

The birefringence of the light guiding plate should preferably be aboutzero. In this case, among the light emitted from the light guidingplate, the polarized light reflected back from one of the abovereflective polarized light selecting devices is transmitted through thelight guiding plate while maintaining its state of polarization.Consequently, this polarized light may be transmitted through the otherreflective polarized light selecting device. As a result, the light ismore efficiently used to provide a brighter display.

By using a light guiding plate with a birefringence of λ/2, the slowaxis of the light guiding plate may be arranged so that it forms anangle of about 45° with each of the reflective polarization axes of eachof the two reflective polarized light selecting devices. Since the slowaxis of the light guiding plate forms an angle of about 45° with each ofthe reflective polarization axes of each of the two reflective polarizedlight selecting devices, the phase of the linearly polarized light,which was a part of the light emitted from the light guiding plate,reflected back from either of the reflective polarized light selectingdevices may be shifted by λ/2, causing the direction of the linearlypolarized light to be rotated by 90°. As a result, even if thereflective polarization axis of each of the two reflective polarizers isdisposed parallel to each other, the light reflected at one of thereflective polarization light selecting devices will definitely betransmitted through the other reflective polarized light selectingdevice.

To address or solve the above, the display unit according to the presentinvention has two liquid crystal panels, a lighting unit, whichilluminates both liquid crystal panels and which is interposed betweenthe two liquid crystal panels, and two reflective polarizers, eachinterposed between the lighting unit and each of the two liquid crystalpanels. In other words, the above transmissive polarization axisrotating devices are composed of liquid crystal panels to form a thinnerdisplay unit (a thinner liquid crystal display unit). In this case, eachof the two reflective polarizers reflects light polarized in apredetermined direction while transmitting light polarized in thedirection that intersects the reflected polarized light. The tworeflective polarizers should preferably be arranged so that thereflective polarization axis of each of the reflective polarizersintersects each other.

The electronic apparatus according to the present invention includes theabove display unit. This electronic apparatus has low electric powerconsumption and a simple and easy structure that allows images to bedisplayed on both the front and the rear surfaces. Furthermore, thedisplays on both surfaces are bright.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional view of the general structure of a liquidcrystal display unit according to a first exemplary embodiment of thepresent invention;

FIG. 2 is a schematic showing the display mechanism of the liquidcrystal display unit of FIG. 1;

FIG. 3 is a cross-sectional view of the general structure of a liquidcrystal display unit according to a third exemplary embodiment of thepresent invention;

FIG. 4 is a schematic showing the display mechanism of the liquidcrystal display unit of FIG. 3;

FIG. 5 is a cross-sectional view of the general structure of a liquidcrystal display unit according to the fourth exemplary embodiment of thepresent invention;

FIG. 6 is a cross-sectional view of the general structure of a liquidcrystal display unit according to the fifth exemplary embodiment of thepresent invention;

FIG. 7 is a perspective view of the folded state of the electronicapparatus according to a sixth exemplary embodiment of the presentinvention;

FIG. 8 is a perspective view of the extended state of the electronicapparatus of FIG. 7.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

Exemplary embodiments of the present invention are described below byreferring to the drawings. To make the drawings describing the exemplaryembodiments more easily viewable, the film thickness and the dimensionalratio of each component included in the drawings are not drawn torepresent the actual dimensions.

First Exemplary Embodiment

FIG. 1 is a cross-sectional view of the general structure of a liquidcrystal display unit according to a first exemplary embodiment of thepresent invention. FIG. 2 illustrates the display mechanism and includesonly the components necessary for the description.

Referring to FIG. 1, a liquid crystal display unit 100, which is anexemplary embodiment according to the present invention, includes a pairof liquid crystal cells 10 and 20 and a backlight (lighting unit) 40used for the liquid crystal cells 10 and 20. The liquid crystal displayunit 100 displays images on the front and the rear surfaces by emittinglight transmitted from the backlight 40 via the liquid crystal cells 10and 20 onto display surfaces 1 and 2. In the liquid crystal cell 10(20), an upper substrate 13 (24) and a lower substrate 14 (23) aredisposed so that they face each other. In the space between the uppersubstrate 13 (24) and the lower substrate 14 (23), a liquid crystallayer 15 (25) composed of liquid crystal sealed therein is interposed.The liquid crystal layers 15 (25) of the liquid crystal cells 10 and 20function as a transmissive polarization axis rotating device to vary thepolarization axis of the transmitted light. The liquid crystal is a TNliquid crystal with a 90° twist angle. In the descriptions of theexemplary embodiments of the present invention, the backlight-40-sidesof the liquid crystal cells 10 and 20 are referred to as the “inner”sides (rear sides) and the opposite sides are referred to as the “outer”sides (front sides).

The backlight 40 is an illumination device disposed adjacent to theinner sides of the liquid crystal cells 10 and 20. The backlight 40includes a light source 42, which is composed of a component, such as acold-cathode tube or an LED, and a light guiding plate 41. The liquidcrystal cells 10 and 20 may be irradiated with light from thelongitudinal sides (the sides facing the liquid crystal cells 10 and 20)of the light guiding plate 41. An organic EL light, for example, may beused as the light source for the backlight 40 allowing light to beemitted from both sides of the backlight 41. In this case, since theorganic EL light is a planar light source, it is not necessary toprovide complex designs and/or processes to uniformly irradiate thefront and the rear surfaces of the light guiding plate 41 with lightemitted from the lateral side.

Also, moire patterns caused by interference with the patterns of thelight guiding plate do not occur frequently. Preferably, the lightguiding plate 41 should be composed of a material with a smallbirefringence such as acrylic resin, PC resin, or polyolefin resin sothat the state of polarization of the reflected polarized light is notaltered easily.

Reflective polarizers 31 and 32, which are reflective polarized lightselecting means, are disposed between each of the two liquid crystalcells 10 and 20 and the light guiding plate 41. The reflectivepolarizers 31 and 32 reflect light linearly polarized in a predetermineddirection while transmitting light linearly polarized in the directionintersecting the reflected polarized light. In this exemplaryembodiment, the reflective polarization axes of the two reflectivepolarizers 31 and 32 intersect each other.

An absorbing polarizer 11(22) is disposed on the outer side of theliquid crystal cell 10 (20), and, similarly, an absorbing polarizer 12(21) is disposed on the backlight-40-side (the inner side) of the liquidcrystal cell 10 (20). In this case, the transmissive polarization axisof the absorbing polarizer 11 (22) is arranged in a directionsubstantially equal to the direction of the transmissive polarizationaxis of the reflective polarizer 31 (32).

Within the liquid crystal cell 10 (20), pixel electrodes (not shown inthe drawings), which are composed of transparent conductive films suchas indium tin oxide (ITO), are disposed on the inside of the lowersubstrate 14 (23), which is composed of a transparent material such asglass or plastic. An alignment film (not shown) composed of polyimide orthe like is deposited to cover the pixel electrode. In this exemplaryembodiment, the lower substrate 14 (23) is composed of a devicesubstrate with pixel switching elements such as TFTs, data lines, andscanning lines. In FIG. 1, however, the pixel switching elements, datalines, and scanning lines are not shown.

Alternatively, on the inside of the upper substrate 13 (24), which iscomposed of a transparent material, such as glass or plastic, a commonelectrode (not shown in the drawings) composed of a transparentconductive film such as indium tin oxide (ITO), and an alignment film(also not shown in the drawings) composed of polyimide or the like arestacked in sequence.

The alignment film on the upper substrate 13 (24) and the lowersubstrate 14 (23) are processed by methods for a horizontal alignment,such as rubbing. The alignment direction of each alignment film for theupper substrate 13 (24) and the lower substrate 14 (23) intersect eachother. The liquid crystal interposed between the upper and lowersubstrates is twisted by 90°. Applying a selective voltage (or turningthe voltage on), aligns the molecules of the liquid crystal layer 15which are disposed between the upper substrate 13 (24) and the lowersubstrate 14 (23), in a direction perpendicular to the surfaces of thesubstrates. In this exemplary embodiment, the TN mode with a 90° twistangle is described as an example. The liquid crystal mode, however, isnot limited to TN liquid crystal mode, and other liquid crystal modesmay be employed as well.

The inner absorbing polarizer 12 (21) and the outer absorbing polarizer11 (22) are arranged so that the directions of their transmissivepolarization axes intersect in the same way as the alignment directionsof the above alignment films intersect. More specifically, as shown inFIG. 2, the transmissive polarization axis of the absorbing innerpolarizer 12 (21) is substantially parallel to the alignment directionof the alignment film formed on the inside of the lower substrate 14(23). At the same time, the axis is substantially parallel to thetransmissive polarization axis of the reflective polarizer 31 (32). Onthe other hand, transmissive polarization axis of the outer absorbingpolarizer 11 (22) intersects (preferably at a 90° angle) thetransmissive polarization axis of the inner absorbing polarizer 12 (21).

The display mechanism of the liquid crystal display unit 100 of thisexemplary embodiment is described below by referring to FIG. 2. As shownon the left side in FIG. 2, the light emitted from the light guidingplate 41 to the first liquid crystal cell 10 (the front liquid crystalcell 10) is selectively polarized at the front reflective polarizer 31.In other words, polarized light (first polarized light) with apolarization axis parallel to the transmissive polarization axis(parallel to the page) of the reflective polarizer 31 is transmittedthrough the reflective polarizer 31, whereas polarized light (secondpolarized light) with a polarization axis parallel to the reflectivepolarization axis (perpendicular to the page) of the reflectivepolarizer 31 is reflected at the reflective polarizer 31.

The first polarized light transmitted through the reflective polarizer31 is then transmitted through the absorbing polarizer 12 (whosetransmissive polarization axis is parallel to the page) on the innerside of the liquid crystal cell 10. Subsequently, the first polarizedlight enters the liquid crystal layer 15, where its polarization axis isrotated according to the application of a selective voltage. When theselective voltage is not applied (or when the voltage is off), the firstpolarized light is transmitted through the liquid crystal layer 15 andits polarization axis is rotated. Then, it is further transmittedthrough the outer absorbing polarizer 11 (whose transmissivepolarization axis is perpendicular to the page), causing the frontdisplay surface (a first display surface) to be bright (or white).Conversely, when the selective voltage is applied (or when the voltageis on), the first polarized light is transmitted through the liquidcrystal layer 15 without its polarization axis being rotated, but thenit is absorbed by the outer absorbing polarizer 11, causing the displayto be dark (or black).

The second polarized light reflected at the reflective polarizer 31 istransmitted through the light guiding plate 41 and then enters the rearreflective polarizer 32. Since, in this case, the light guiding plate 41is composed of a material with a small birefringence (preferable with nobirefringence), as mentioned above, the polarization axis of the secondpolarized light is not rotated as it is transmitted through the lightguiding plate 41.

The reflective polarization axis of the rear reflective polarizer 32 isset to intersect the reflective polarization axis of the frontreflective polarizer 31. Therefore, the second polarized light thatenters the rear reflective polarizer 32 is transmitted through thisreflective polarizer 32 and then transmitted through the inner absorbingpolarizer 21 of the second liquid crystal cell 20 (the rear liquidcrystal cell) to enter the liquid crystal layer 25. At this liquidcrystal layer 25, the polarization axis is rotated according to theapplication of the selective voltage. When the selective voltage is notapplied (or when the voltage is off), the second polarized light istransmitted through the liquid crystal layer 25 and its polarizationaxis is rotated. Then it is transmitted through the outer absorbingpolarizer 22, (whose transmissive polarization axis is parallel thepage), causing the rear display surface (a second display surface) to bebright (or white). Conversely, when the selective voltage is applied (orwhen the voltage is on), the second polarized light is transmittedthrough the liquid crystal layer 25 without its polarization axis beingrotated, but then it is absorbed by the outer absorbing polarizer 22,causing the display to be dark (or black).

Similarly, as shown on the right side in FIG. 2, the light emitted fromthe light guiding plate 41 to the rear liquid crystal cell 20 is alsoselectively polarized at the rear reflective polarizer 31. The lighttransmitted through the reflective polarizer 32 is third polarizedlight, and the light reflected at the reflective polarizer 32 is fourthpolarized light. The transmitted third polarized light can be emitted tothe rear display surface (the second display surface) via the liquidcrystal cell 20. Furthermore, the reflected fourth polarized light canbe emitted to the front display surface (the first display surface) viathe front reflective polarizer 31 and the liquid crystal cell 10.Consequently, images can be displayed on both the front and the rearsurfaces of the display unit.

As described above, in this exemplary embodiment, the polarized lightreflected at one of the reflective polarizers (for instance, thereflective polarizer 31) may be transmitted through the other reflectivepolarizer (for instance, the reflective polarizer 32). As a result, thelight utilization efficiency is enhanced, and the brightness of thefront and the rear surfaces of the liquid crystal panel increases.Moreover, since two liquid crystal panels may be illuminated with onelight guiding plate, the liquid crystal display unit becomes thin, andthe number of parts necessary for the unit may be reduced.

Second Exemplary Embodiment

In this exemplary embodiment, the birefringence of the light guidingplate 41 was about zero. Alternatively, the birefringence of the lightguiding plate 41 may be λ/2, while the slow axis of the light guidingplate 41 may form an angle of about 45° with the reflective polarizationaxis of each of the reflective polarizers 31 and 32. Moreover, thereflective polarization axis of each of the reflective polarizers 31 and32 may be parallel to each other. When the linearly polarized light,which was part of the light emitted from the light guiding plate 41 andwhich was reflected back from either one of the reflective polarizers 31and 32, is transmitted through the light guiding plate 41, whose slowaxis, as described above, intersects the reflective polarization axis ofeach of the reflective polarizers 31 and 32 at 45°, the phase of therespective linearly polarized light is shifted by λ/2 and the directionof the polarization of the respective linearly polarized light isrotated by 90°. Since the reflective polarization axes of the reflectivepolarizers 31 and 32 are parallel, the linearly polarized lightreflected at one of the reflective polarizers is rotated by 90° at thelight guiding plate and becomes linearly polarized light that isperpendicular to the reflective axis of the other reflective polarizer(i.e., parallel to the transmissive axis). As a result, the reflectedpolarized light is assured to be transmitted through the otherreflective polarizer. As described here, by controlling thebirefringence of the light guiding plate and the directions of the axes,the angle of the reflective polarization axis of each reflectivepolarizer may be rotated by 90°.

Third Exemplary Embodiment

FIG. 3 is a cross-sectional view of the overall structure of the liquidcrystal display unit as a third exemplary embodiment. FIG. 4 shows itsdisplay mechanism and shows only the components necessary for thedescription. For components that are indicated by the same referencenumerals as the liquid crystal display unit 100 of the first exemplaryembodiment shown in FIG. 1, the structures of these components are thesame as the ones shown in FIG. 1 unless otherwise stated.

Similar to the first exemplary embodiment in FIG. 1, the liquid crystaldisplay unit 200, as shown in FIG. 3, includes two liquid crystal cells10 and 20 and a backlight (lighting unit) 40 used for the liquid crystalcells 10 and 20. The liquid crystal display unit 200 displays images onboth the front and the rear surfaces by emitting light transmitted fromthe backlight 40 via the liquid crystal cells 10 and 20 onto the displaysurfaces 1 and 2.

Reflective polarizers 35 and 36, which are reflective polarized lightselecting devices, are each interposed between the backlight 40 and eachof the liquid crystal cells 10 and 20, respectively. The reflectivepolarizers 35 and 36 are composed of cholesteric liquid crystal. Thesepolarizers reflect circularly polarized light rotating in apredetermined direction, while transmitting circularly polarized lightrotating in the opposite direction. In this exemplary embodiment, bothof the reflective polarizers 35 and 36 are set to reflect the circularlypolarized light rotating in the same rotative direction. Morespecifically, the circularly polarized light reflected at the reflectivepolarizer 35 may be transmitted through the other reflective polarizer36.

On the inner side of the liquid crystal cell 10 (20), i.e., on the innerside of the absorbing polarizer 12 (21), a λ/4 retardation film 16 (26)is disposed. The retardation axis of the λ/4 retardation film 16 (26) isarranged so that the circularly polarized light, after being transmittedthrough the λ/4 retardation film 16 (26), becomes linearly polarizedlight that may be transmitted through the absorbing polarizer 12 (21).

In the below, the display mechanism of the liquid crystal display unit200 according to this exemplary embodiment is described by referring toFIG. 4. The rotative directions of the circularly polarized light arethe rotative directions viewed from the direction opposite to that inwhich the light travels.

As shown on the left in FIG. 4, the light emitted from the light guidingplate 41 to the first liquid crystal cell 10 (the front liquid crystalcells 10) is selectively polarized at the front reflective polarizer 35.More specifically, at the reflective polarizer 35, theleft-circularly-polarized light (first polarized light) is transmittedthrough the reflective polarizer 35, while theright-circularly-polarized light (second polarized light) is reflectedat the reflective polarizer 35. After being reflected, the reflectedcircularly polarized light becomes left-circularly-polarized light(third polarized light).

After the first polarized light is transmitted through the reflectivepolarizer 35, it is transmitted through the k/4 retardation film 16disposed on the inner side of the liquid crystal cell 10 and becomeslinearly polarized light with a polarization axis parallel to the page.This linearly polarized light is further transmitted through theabsorbing polarizer 12 (whose transmissive polarization axis is parallelto the page) and then enters the liquid crystal layer 15. At the liquidcrystal layer 15, the polarization axis is rotated according to theapplication of the selective voltage. When the selective voltage is notapplied (or when the voltage is off), the first polarized light istransmitted through the liquid crystal layer 15 and its polarizationaxis is rotated. Then it is transmitted through the outer absorbingpolarizer 11 (whose transmissive polarization axis is perpendicular tothe page), causing the front display surface (a first display surface)to be bright (or white). Conversely, when the selective voltage isapplied (or when the voltage is on), the first polarized light istransmitted through the liquid crystal layer 15 without its polarizationaxis being rotated, but then it is absorbed by the outer absorbingpolarizer 11, causing the display to be dark (or black).

The second polarized light reflected at the reflective polarizer 35 istransmitted through the light guiding plate 41 and then enters the rearreflective polarizer 36. Since the light guiding plate 41 is composed ofa material with a small birefringence (preferably with nobirefringence), as mentioned above, the rotative direction of thecircularly polarized light is not altered when it is transmitted throughthe light guiding plate 41.

The rear reflective polarizer 36 is arranged so that the rotativedirection of the reflective light intersects the rotative direction ofthe light reflected at the front reflective polarizer 35. Thus, thesecond polarized light that entered the rear reflective polarizer 36 istransmitted through the reflective polarizer 36 and then through the λ/4retardation film 26 disposed on the inner side of the second liquidcrystal cell 20 (the rear liquid crystal cell). At the λ/4 retardationfilm 26, the second polarized light is converted into linearly polarizedlight with a polarization axis parallel to the page. This linearlypolarized light is further transmitted through the absorbing polarizer21(whose transmissive polarization axis is parallel to the page) andenters the liquid crystal layer 25. At this liquid crystal layer 25, thepolarization axis is rotated according to the application of theselective voltage.

When the selective voltage is not applied (or when voltage is off), thesecond polarized light is transmitted through the liquid crystal layer25 and its polarization axis is rotated. Then, it is transmitted throughthe outer absorbing polarizer 22 (whose transmissive polarization axisis perpendicular to the page), causing the rear display surface (asecond display surface) to be bright (or white). Conversely, when theselective voltage is applied (or when the voltage is on), the secondpolarized light is transmitted through the liquid crystal layer 25without its polarization axis being rotated, but then it is absorbed bythe outer absorbing polarizer 22, causing the display to be dark (orblack).

The light emitted from the light guiding plate 41 to the rear liquidcrystal cell 20 is also selectively polarized at the rear reflectivepolarizer 36, as in the right side in FIG. 2. Left-circularly-polarizedlight (the third polarized light) is transmitted through the reflectivepolarizer 36, while right-circularly-polarized light (fourth polarizedlight) is reflected at the reflective polarizer 36. This reflected lightbecomes left-circularly-polarized light (the first polarized light).

The third polarized light that is transmitted through the reflectivepolarizer 36 may be emitted onto the rear display surface (the seconddisplay surface) via the liquid crystal cell 20. The reflected firstpolarized light may be emitted onto the front display surface (the firstdisplay surface) via the front reflective polarizer 35 and the liquidcrystal cell 10. As a result, images may be displayed on both the frontand the rear surfaces.

As described above, this exemplary embodiment also includes reflectivepolarizers that are composed of a cholesteric liquid crystal utilizingthe effect of dichroism of the circularly polarized light. Byselectively polarizing the circularly polarized light according to itsrotative direction, the polarized light reflected at one of thereflective polarizers (for instance, the reflective polarizer 35) may betransmitted through the opposite reflective polarizer (for instance, thereflective polarizer 36). As a result, light utilization efficiency isenhanced, and the brightness of the front and the rear surfaces of thepanel increases. Moreover, since two liquid crystal panels may beilluminated with one light guiding plate, the liquid crystal displayunit becomes thin, and the number of parts necessary for the unit may bereduced.

Fourth Exemplary Embodiment

FIG. 5 is a cross-sectional view of the overall structure of the liquidcrystal display unit according to a fourth exemplary embodiment. Theliquid crystal display unit has exactly the same structure as the firstexemplary embodiment shown in FIG. 1, except that a scattering layer 51(52) composed of a film with a roughened surface is interposed betweenthe liquid crystal cell 10 (20) and the reflective polarizer 35 (36).Adopting this structure may reduce or prevent specular reflection at thereflective polarizers. Also, the roughened surfaces reduce or preventthe generation of Newton rings.

For this exemplary embodiment, a film with a roughened surface is usedfor the scattering layers 51 and 52. Instead, an adhesive scatteringlayer with an adhesive film including beads that function as scatterersmay be used. By using this adhesive scattering layer, the liquid crystalcells 10 and 20 may be bonded to the reflective polarizers 31 and 32,respectively. As a result, in addition to the above mentioned effects,the entire liquid crystal display unit becomes even thinner. Also,displacement caused by vibration and/or shock, and/or entry of foreignmaterials may be reduced or prevented.

Fifth Exemplary Embodiment

FIG. 6 is a cross-sectional view of the overall structure of the liquidcrystal display unit according to a fifth exemplary embodiment. As shownin FIG. 6, the sizes of the display areas of the liquid crystal cells 10and 20 differ. In comparison with the size of the display area of theliquid crystal cell 10, the display area of the liquid crystal cell 20is smaller. A light shielding layer 61 composed of a light absorbingmaterial is interposed between the liquid crystal cell 20 and thereflective polarizer 32. Except for the size difference of the displayareas, the structure of the liquid crystal display is exactly the sameas the first exemplary embodiment shown in FIG. 1.

By adopting the above structure, degradation of the display qualitycaused by the shadow of the liquid crystal cell 20 visible from theliquid-crystal-cell- 10-side of the display unit may be reduced.

Sixth Exemplary Embodiment

An electronic apparatus according to a sixth exemplary embodiment of thepresent invention is described by referring to FIGS. 7 and 8. A cellularphone 1000 is an exemplary embodiment of the electronic apparatusaccording to the present invention. The cellular phone 1000 is afoldable cellular phone that may take a folded state, as shown in FIG.7, and an extended state, as shown in FIG. 8. The cellular phone 1000includes a body 1001 and a display 1002.

The above liquid crystal display unit is disposed inside the display1002. Display images are visible on the front display surface 1003 andthe rear display surface 1004 on the display 1002. For this cellularphone 1000, bright display images become visible on the front displaysurface 1003 and/or the rear display surface 1004 in response to thevarious operations and different states of the apparatus, including, inparticular, the change from the folded state to the extended state, andvice versa.

The liquid crystal display unit related to the display unit according tothe present invention and the exemplary embodiment of the cellular phonerelated to the electronic apparatus according to the present inventionare both described above. However, the present invention is not limitedto the descriptions above and may be modified in various ways as long asthe modifications do not deviate from the spirit of the presentinvention.

1. A display device comprising: a first transmissive polarization axis rotating device; a second transmissive polarization axis rotating device having a display area size that is smaller than a display area size of the first transmissive polarization axis rotating device; an illumination device disposed between the first transmissive polarization axis rotating device and the second transmissive polarization axis rotating device, the illumination device having a larger area in plan view than the display area of the first transmissive polarization axis rotating device, the illumination device being capable of illuminating the display area of the first transmissive polarization axis rotating device and the display area of the second transmissive polarization axis rotating device; a light scattering layer disposed between the illumination device and the second transmissive polarization axis rotating device; and a light shielding layer interposed between the second transmissive polarization axis rotating device and the illumination device, the light shielding layer including an opening that enables the display area of the second transmissive polarization axis rotating device to be a light transmissive region.
 2. The display device according to claim 1, the light shielding layer being disposed on both sides of the opening in cross-sectional view of the display device.
 3. The display device according to claim 1, the scattering layer being composed of a film with a roughened surface or an adhesive film with a light scattering function.
 4. The display device according to claim 1, the illumination device including: a light source; and a light guiding plate that guides light from the light source, the light guiding plate having a birefringence of about zero.
 5. A foldable cellular phone, comprising: the display device according to claim 1; and a body paired with the display device.
 6. An electronic device comprising the display device according to claim
 1. 7. A display device comprising: a first transmissive polarization axis rotating device; a second transmissive polarization axis rotating device having a display area size that is smaller than a display area size of the first transmissive polarization axis rotating device; an illumination device disposed between the first transmissive polarization axis rotating device and the second transmissive polarization axis rotating device, the illumination device having a larger area in plan view than the display area of the first transmissive polarization axis rotating device, the illumination device being capable of illuminating the display area of the first transmissive polarization axis rotating device and the display area of the second transmissive polarization axis rotating device; a light scattering layer disposed between the illumination device and the first transmissive polarization axis rotating device; and a light shielding layer interposed between the second transmissive polarization axis rotating device and the illumination device, the light shielding layer including an opening that enables the display area of the second transmissive polarization axis rotating device to be a light transmissive region.
 8. The display device according to claim 7, the light shielding layer being disposed on both sides of the opening in cross-sectional view of the display device.
 9. The display device according to claim 7, the scattering layer being composed of a film with a roughened surface or an adhesive film with a light scattering function.
 10. The display device according to claim 7, the illumination device including: a light source; and a light guiding plate that guides light from the light source, the light guiding plate having a birefringence of about zero.
 11. A foldable cellular phone, comprising: the display device according to claim 7; and a body paired with the display device.
 12. An electronic device comprising the display device according to claim
 7. 13. A display device comprising: a first transmissive polarization axis rotating device; a second transmissive polarization axis rotating device having a display area size that is smaller than a display area size of the first transmissive polarization axis rotating device; an illumination device disposed between the first transmissive polarization axis rotating device and the second transmissive polarization axis rotating device, the illumination device having a larger area in plan view than the display area of the first transmissive polarization axis rotating device, the illumination device being capable of illuminating the display area of the first transmissive polarization axis rotating device and the display area of the second transmissive polarization axis rotating device; a first light scattering layer disposed between the illumination device and the first transmissive polarization axis rotating device; a second light scattering layer disposed between the illumination device and the second transmissive polarization axis rotating device; and a light shielding layer interposed between the second transmissive polarization axis rotating device and the illumination device, the light shielding layer including an opening that enables the display area of the second transmissive polarization axis rotating device to be a light transmissive region.
 14. The display device according to claim 13, the light shielding layer being disposed on both sides of the opening in cross-sectional view of the display device.
 15. The display device according to claim 13, the scattering layers being composed of a film with a roughened surface or an adhesive film with a light scattering function.
 16. The display device according to claim 13, the illumination device including: a light source; and a light guiding plate that guides light from the light source, the light guiding plate having a birefringence of about zero.
 17. A foldable cellular phone, comprising: the display device according to claim 13; and a body paired with the display device.
 18. An electronic device comprising the display device according to claim
 13. 19. A display device comprising: a first liquid crystal panel; a second liquid crystal panel having a display area size that is smaller than a display area size of the first liquid crystal panel; an illumination device disposed between the first liquid crystal panel and the second liquid crystal panel, the illumination device having a larger area in plan view than the display area of the first liquid crystal panel, the illumination device being capable of illuminating the display area of the first liquid crystal panel and the display area of the second liquid crystal panel; a light scattering layer disposed between the illumination device and the second liquid crystal panel; and a light shielding layer interposed between the second liquid crystal panel and the illumination device, the light shielding layer including an opening that enables the display area of the second liquid crystal panel to be a light transmissive region.
 20. The display device according to claim 19, the light shielding layer being disposed on both sides of the opening in cross-sectional view of the display device.
 21. The display device according to claim 19, the scattering layer being composed of a film with a roughened surface or an adhesive film with a light scattering function.
 22. The display device according to claim 19, the illumination device including: a light source; and a light guiding plate that guides light from the light source, the light guiding plate having a birefringence of about zero.
 23. The display device according to claim 19, the illumination device including an organic EL light that emits light from two surfaces to radiate a surface of the first liquid crystal panel and a surface of the second liquid crystal panel with light.
 24. A foldable cellular phone, comprising: the display device according to claim 19; and a body paired with the display device.
 25. An electronic device comprising the display device according to claim
 19. 26. A display device comprising: a first liquid crystal panel; a second liquid crystal panel having a display area size that is smaller than a display area size of the first liquid crystal panel; an illumination device disposed between the first liquid crystal panel and the second liquid crystal panel, the illumination device having a larger area in plan view than the display area of the first liquid crystal panel, the illumination device being capable of illuminating the display area of the first liquid crystal panel and the display area of the second liquid crystal panel; a light scattering layer disposed between the illumination device and the first liquid crystal panel; and a light shielding layer interposed between the second liquid crystal panel and the illumination device, the light shielding layer including an opening that enables the display area of the second liquid crystal panel to be a light transmissive region.
 27. The display device according to claim 26, the light shielding layer being disposed on both sides of the opening in cross-sectional view of the display device.
 28. The display device according to claim 26, the scattering layer being composed of a film with a roughened surface or an adhesive film with a light scattering function.
 29. The display device according to claim 26, the illumination device including: a light source; and a light guiding plate that guides light from the light source, the light guiding plate having a birefringence of about zero.
 30. The display device according to claim 26, the illumination device including an organic EL light that emits light from two surfaces to radiate a surface of the first liquid crystal panel and a surface of the second liquid crystal panel with light.
 31. A foldable cellular phone, comprising: the display device according to claim 26; and a body paired with the display device.
 32. An electronic device comprising the display device according to claim
 26. 33. A display device comprising: a first liquid crystal panel; a second liquid crystal panel having a display area size that is smaller than a display area size of the first liquid crystal panel; an illumination device disposed between the first liquid crystal panel and the second liquid crystal panel, the illumination device having a larger area in plan view than the display area of the first liquid crystal panel, the illumination device being capable of illuminating the display area of the first liquid crystal panel and the display area of the second liquid crystal panel; a first light scattering layer disposed between the illumination device and the first liquid crystal panel; a second light scattering layer disposed between the illumination device and the second liquid crystal panel; and a light shielding layer interposed between the second liquid crystal panel and the illumination device, the light shielding layer including an opening that enables the display area of the second liquid crystal panel to be a light transmissive region.
 34. The display device according to claim 33, the light shielding layer being disposed on both sides of the opening in cross-sectional view of the display device.
 35. The display device according to claim 33, the scattering layers being composed of a film with a roughened surface or an adhesive film with a light scattering function.
 36. The display device according to claim 33, the illumination device including: a light source; and a light guiding plate that guides light from the light source, the light guiding plate having a birefringence of about zero.
 37. The display device according to claim 33, the illumination device including an organic EL light that emits light from two surfaces to radiate a surface of the first liquid crystal panel and a surface of the second liquid crystal panel with light.
 38. A foldable cellular phone, comprising: the display device according to claim 33; and a body paired with the display device.
 39. An electronic device comprising the display device according to claim
 33. 